... idk if anyone ever pays attention to my rants on george.... but....
i like to sabotage his life, he likes to sabotage mine. yet we're still bffs.
:D
i kill him on a daily basis. i should ask him to get a cr so i can prove it.

Defeat Means Friendship is an overly used plot device that lets character's alignments shift without too much transaction necessary. Similar to Because You Were kind To Me in the way it presents loyalties and priorities being changed. Albeit the latter is completely, DMF is rarely a complete change of a characters priorities.

The ongoing series (Bleach, Naruto, Reborn, Fairy Tail, Gintama, et cetera) are the largest offenders of this cliche. Though yes, dragonball was also a huge part of this trope in its time.

Its a plot convenience you see in anime and manga all the time, and the amount..... yeah, et cetera et cetera. I'll let someone else take it from here. God I hate explanations.

1) The history of the genre. Since it was done by series that were successful during their time, writers might be attempting to mimic them in hopes of achieving similar success. Understandable if unoriginal. Fact is people like what's familiar to them which is why a lot of popular shows, movies, games, books, etc. can be found using similar elements as other popular works that came before them. To take a different direction using that last point, one has to use what is known to create something new. Steve Jobs (or, more specifically, Apple's R&D team) couldn't create an iPhone without knowing how computers and phones worked since that knowledge is necessary to merge the two technologies.

2) Popularity of the villains. Everyone loves a bad guy. Be them the monsterous type that, fights like an animal and kills without mercy (Examples Xenomorphs, most slaher movies villians), the suave and calculating man who can always stay one step ahead (Examples Light), the villain who is just so evil and crazy to a comic book extreme that you just got to love them (Example Joker), etc. So, if the villain they had fight the hero was very popular, then the writer thinks he can only help his sales by keeping him in the story. It'd get boring if he stayed a villain, so a rival or uneasy ally is a perfect fit for him. To be fair to the writers who do this, it often works. But, it should be noted, if a valid reason for the shift cannot be provided to the audience, it can have the inverse affect.